Electro-optical recording and visual display systems



Nov. 30, 1 F. A. SCHWERTZ ELECTRO-OPTICAL RECORDING AND VISUAL DISPLAY SYSTEMS Filed Feb. 4, 1957 m E S A cu m m5 N wW 6 M D W E C R M R 1 I 1 1 IIZIZIIIIIZZI I N V EN TOR. EQEDHe/cK A. SCHWERTZ fiz /wh w United States Patent 3,221,335 ELECTRO-OPTICAL RECORDING AND VISUAL DISPLAY SYSTEMS Frederick A. Schwertz, Pittsford, N.Y., assignor to Xerox ggory aoration, Rochester, N.Y., a corporation of New ork Filed Feb. 4, 1957, Ser. No. 638,008 1 Claim. (Cl. 346-74) The present invention relates to the electrostatic recording of signal intelligence and the visual presentation thereof.

In my copending patent application entitled Electrostatic Recording of Information, filed Nov. 20, 1956 and assigned Serial No. 623,327, now abandoned, there is disclosed an electrostatic image-forming process for recording digital or analog information at high speed. In this process, by means of shaped electrodes, symbols or characters are impressed as electrostatic charges on an insulating web. The web is electrically pre-strcssed to a condition below the critical stress value. Transfer of the character or symbol from the shaped electrode is effected by the use of a relatively small triggering pulse which raises the electric field above critical stress to produce a field discharge. This discharge action in the air space between the shaped electrodes and the insulating web gives rise to the formation of an electrostatic charge pattern of the symbol on the web. Electronic switching circuits are associated with the electrostatic electrodes to supply triggering pulses thereto in accordance with information received electrically from a digital computer or other signal or other signal source.

In the system disclosed in the above-identified application, the recording pulses are applied to an electrode structure which is mounted above a moving web of insulating material and as the web advances, successive lines of information are electrostatically impressed thereon. Thus the rate at which recording takes place is ultimately limited by the speed at which the web may be moved.

Accordingly, it is a major object of this invention to provide an electrostatic recording system without moving parts wherein signal intelligence may be impressed on an immobile surface.

More specifically, it is an object of the invention to provide an electrostatic electrode structure adapted to impress successive lines of data on an insulating surface and requiring no mechanical movement either of the electrodes or of the insulating surface associated therewith. A salient advantage of the invention resides in the fact that digital or analog data may be recorded at extremely high speeds, the system being altogether free of mechanical limitations.

Also an object of the invention is to provide an electrostatic technique wherein an electrostatically charged area is rendered immediately visible. Because of the extreme rapidity of the electrostatic recording system computational information or other forms of intelligence are displayed at speeds comparable to that of a cathode rayscanning beam.

A further object of the invention is to provide a display device wherein images are formed by electrostatically exciting an electroluminescent surface by means of shaped electrodes, the surface being movable relative to the electrodes. Thus information may be recorded at high speed and rendered immediately visible.

Yet another object of the invention is to provide a television display device wherein an electroluminescent screen is electrostatically excited. A significant feature of the invention lies in the use of an electrostatic electrode structure adapted to effect dot sequential scanning of the screen to produce luminous images thereon.

3,221,335 Patented Nov. 30,1965

For a better understanding of the invention as well as other objects and further features thereof, reference is had to the following detailed description to be read in connection with the accompanying drawings.

In the drawings:

FIG. 1 is a schematic diagram illustrating in perspective an electrostatic recording apparatus in accordance with the invention for applying information on a motionless medium.

FIG. 2 is a sectional view of the electrode structure shown in FIG. 1.

FIG. 3 shows in perspective another embodiment of the invention for the visual display of electrostatically recorded intelligence.

FIG. 4 is a front view of the display screen shown in FIG. 3.

FIG. 5 is a sectional view of the display screen shown in FIG. 3.

FIG. 6 is a perspective view of a modified form of display screen.

FIG. 7 is a section taken through the screen shown in FIG. 6.

Referring now to the drawings and more particularly to FIG. 1, there is shown an arrangement adapted to record information electrostatically on a motionless insulating meduim 10. The insulating medium may be constituted by a plastic-coated paper or any other dielectric sheet having a sufiiciently high resistance to hold. an electrostatic image for a period of such duration as to permit subsequent utilization of the image by transfer to another surface or by development. Among the materials suitable for this purpose are terephthalate, cellulose acetate and polyethylene.

The medium 10 is interposed between two orthogonally arranged sets of electrodes, generally designated by numerals 11 and 12. Electrode set 11 is constituted by a plurality of equi-spaced conductors 11a, 11b, 11c and 11d disposed in parallel relation and embedded or printed by electrochemical techniques on the upper face of an insulating plate 13 which may be formed of glass or a rigid plastic material. Disposed above electrode set 11 and spaced therefrom is electrode set 12 which is similarly constituted by a plurality of conductors 12a, 12b, 12c and 12d extending perpendicularly with respect to the conductors of set 11 and embedded or printed on the under face of an insulating plate 14. While for purposes of simplicity each electrode set is illustrated as being composed of four wires, it is to be understood that in practice a greater number may be used, as desired.

In accordance with the invention, if any two conductors, one from each set, are electrically activated, an electrostatic point image can be transferred to the paper at a position corresponding to the point of virtual intersection of the activated conductors. For example, if a potential of the proper magnitude is applied between conductor 12c and 11a, an electrostatic image will be formed at the point on medium 10 aligned with the point of virtual intersection 12c-11a as indicated by arrow 15. The upper conductors at the points of intersection with the lower conductors may be shaped rather than made linear so as to impress similarly shaped images on the paper.

In its broad aspect the problem of recording data on the medium involves the distribution of symbols thereon in an arbitrary manner. Let us assume that data expressed by a four place binary system is to be recorded, each binary value being indicated by a pulse representing 1 or by a blank representing 0. Let us further assume that these binary pulses are yielded simultaneously or singly in the output of a digital computer represented by block 16, each binary value being applied to a respective conductor of electrode set 11. The application of the pulses to the conductors may be serially in a train or parallel in time.

At the instant the binary information is applied to the electrode set 11, the operation of a timing pulse generator 17 is initiated to actuate a ring-counter time base circuit 18 having stages A, B, C and D arranged in cascade relation and connected respectively to electrodes 11a, lib, 11c and 11d. Each time the binary pulses representing a given number are applied to the electrodes 12a to 12d, a stepping action occurs in the ring counter 18 which is arranged to have the activated stage establish a high voltage on the associated conductor of set 11, the other stages remaining at a comparatively low voltage. Thus with successive operations, the high voltage is shifted from stage A to B to C to D, the high voltage being relative to ground.

The pulse voltages applied to conductors 12a to 12b are also relative to ground, the pulses being of a polarity with respect to the voltage from the ring counter as to have an additive field effect. The combined magnitude of the pulse and the high voltage is above the critical field stress value, which defines that value of electric field strength at which field discharge or breakdown occurs. It has been found that when there is such a discharge, a transfer .or charge migration through the gap between an electrode and the insulating medium occurs. If, on the other hand, the electric stress is below the critical level, dielectric breakdown is not eifected and there is no charge transfer.

Underlying the present invention is the fact that the magnitude of voltage applied selectively to electrodes 11a to 11d by ring counter 18 is below the critical value, the stress being raised above critical value only when a pulse is applied to a conductor 12a to 12d of the other electrode set. The region at which this above-critical value stress is exerted is that extending between the activated conductors of the two sets at their virtual point of intersection and at no other point. Hence a charge migration will occur at the virtual intersection point to form an electrostatic point pattern on the insulating medium.

The manner in which data is impressed line by line on medium is as follows: Let us at the outset assume that the binary pulses representing the first number to be printed is applied to conductors 12a to 12d. At that moment, stage A of the ring counter 18 is activated and field discharges occur at those intersections of conductors 12a to 12d and conductor 11a at which voltage pulses are present. In this way a first data line is laid down electrostatically. The next set of binary pulses is also applied to conductor-s 12a to 12d but at this time stage B of the counter 18 is activated to apply the high voltage to conductor 11b. Hence field discharges now occur at those intersections of conductors 12a to 12d and conductor 11b at which voltage pulses are present, thereby laying down a second line. The same process is repeated for the intersections of conductors 12a to 12d and con ductors 11c and 11d, thereby printing the third and fourth line. Of course the number of lines depends on the number of counter stages and associated conductors and this may be augmented as desired.

In the event that it becomes necessary to record a transient voltage change rather than digital values, this can be accomplished by first translating the transient voltage in a suitable converter, such as a digital voltmeter into its digital equivalent. The transient voltage is treated as an analog voltage which varies in time. The digital number yielded by the converter is set up in a number register, the number in the register is then decoded by a line selection matrix connected to the conductors 12a to 12d to place a pulse on only one of the several conductors. The higher the instantaneous value .of the transient voltage, the higher the number of the conductor which is activated. Thus in operation, the first pulse is recorded on the first line, the second pulse on the second line, etc., to produce a trace of dots representing the varying value of the transient voltage. A

more detailed description of the apparatus for translating voltages which vary in time to digital values and for selectively exciting a plurality of lines is disclosed in the above-cited copending application.

An arrangement similar to that shown in FIG. 1 but using olar or any other set of orthogonal coordinates rather than the Cartesian coordinates as defined by electrode sets 11 and 1.2 may be employed for recording purposes. Alternatively, the digital information may be fed to both sets of electrodes to yield an xy plot from which the time factor is eliminated.

After the medium 10 is charged with the data to be recorded, it may be removed from between the electrode sets and developed and fixed in the manner customary in the Xerographic art. Development is accomplished by the deposition of finely divided powder on the surface of the medium, the powder adhering to the charged areas. Thereafter the charge pattern is fixed by fusing the powder on the surface of a print to which the powder pattern has been transferred. A detailed description of the Xerogaphic developing and fusing technique and the apparatus involved therein may be found in the Carlson Patent No. 2,297,691.

As pointed out previously, a charge pattern is formed on the insulating web when a field discharge is produced in the air gap between the insulating medium and the pulsed electrode. The nature of the field discharge is such that when critical stress is attained, ions which normally are present in the gap are accelerated into collisions with nearby air molecules, thereby generating additional ions which similarly collide wit-h molecules to create more ions, this action being cumulative. Charges are also released from the surfaces defining the gap by collisions with these surfaces by the moving ions. The travelling ions so produced deposit on the surface controlled by the electric field.

It has been found that the electric field discharges produced in the air gap may be used to excite electroluminescent phosphors whereby a direct display of information is possible.

In FIGS. 3, 4 and 5 there is shown an information display device for exhibiting Arabic numerals including an electrode structure 19 in which conductive electrode elements 20 shaped as numerals 1 to 9 are inlaid at spaced positions on an insulating plate. The individual elements are connected by lines to an electronic or electromechanical switching circuit 21 adapted to apply pulses selectively thereto. The operation of the switching circuit may be controlled by a binary converter whereby binary pulses representing given numbers effect the excitation of the appropriate line.

The display screen, as shown separately in FIG. 5, is constituted by a dielectric plate 22 in which electroluminescent phosphor material is embedded, the material being responsive to the action of an alternating or pulsed electrical field. Electroluminescent plate 22 is supported on a planar transparent base 23 which may be of glass or of a suitable plastic, the face of the base being coated with a conductive layer 24 of transparent material. This conductive coating may for example take the form of an extremely thin coating of metal, such as aluminum, applied to the base by the vacuum evaporation technique.

The shaped metal electrode 20 is supported above the electroluminescent plate 22' and is spaced therefrom a few thousandths of an inch to form an air gap. A voltage source 25 is connected between the shaped electrode 20 and the conductor coating 24 through a switch 26, whereby when the switch is momentarily closed, a pulsed potential is impressed across the air gap and a field discharge occurs which excites the phosphor in plate 22. Since this plate comprises electroluminescent phosphors bound in a lastic matrix, an outline light pulse is produced in the pattern of the shaped electrode, this result arising from the fact that such electroluminescent layers rebound to the action of an alternating or pulsed field.

The voltages and air gap distances are those usually associated with electrostatic recording apparatus. If desired, a pre-stressing potential may be applied such that the electrode field is normally below the critical stress value whereby when a pulse is applied to the shaped electrode, the pulse magnitude raises the field above critical stress to cause a field discharge.

Since the conductive layer 24 is transparent, it in no way interferes with the transmission of light from the luminescent plate 22, and the characters on the plate may readily be observed by a viewer. While the shaped electrode 20 has been described as being in the form of Arabic numerals, it is obvious that any other shape may be used to render visible any desired form of intelligence.

The importance of the optical display arrangement shown herein resides in the fact that relative motion may exist between the actuating electrode and the electroluminescent layer by virtue of the existence of the small air gap therebetween. Thus all of the concepts of electrostatic recording involving a moving medium are applicable here. For example, in a teleprinter of the type disclosed in the above identified copending application wherein a spinning electrode character cylinder is used in conjunction with a moving recording web of insulating material, the insulating web may be replaced by a moving luminescent surface to atford directly visible readings.

It is also possible to employ the same principle to produce what in efiect is the equivalent of a flat cathode ray tube. In the display screen in accordance with the invention, as shown in FIGS. 6 and 7, a grid or matrix of mutually perpendicular conductors are formed on insulating plates, very much in the manner disclosed in connection with FIG. 1. However, the surface on which the charge is deposited carries a phosphorescent layer which is excited at the point of virtual intersection of the wires. By suitable time base circuits, successive points of intersection may be excited across a horizontal line on the screen and after each horizontal line is traversed, the scanning action may be shifted vertically to effect a scanning action along a succeeding horizontal line in the manner analogous to the action of the deflection system in a cathode ray tube.

The screen in FIG. 7 is constituted by a dielectric plate 27 having luminescent phosphors embedded therein. The plate is supported on an insulating base 28 on whose face is coated or otherwise printed conductive lines 29 of transparent material, the lines being disposed vertically in parallel relation to constitute one set of electrodes.

Disposed adjacent luminescent plate 28 is a second insulating plate 30 of like dimensions on which is printed or coated conductive lines 31 which extend horizontally, the lines 31 representing a second and orthogonal set of electrodes. The operation of the display device is similar to that disclosed in connection with FIG. 1 and when the voltage between two electrodes at their virtual point of intersection is above critical stress, a field discharge occurs. This field discharge excites the phosphor at the corresponding point to produce a visible display.

The display structures in FIGS. 6 and 7 may be used to present video images. This is accomplished by connecting the horizontal conductors 31 to a time base circuit 32 which may be in the form of a ring counter having a like number of stages. Timing pulses from a source 33 eifect step advance of the ring counter upon completion of each horizontal scan of the screen, thereby shifting the scan vertically one line down.

The horizontal scanning action is effected by conductors 29 which are connected to a video signal circuit 34 including a line selector and associated sampling circuits adapted to apply pulses serially to the conductors, the magnitude of each pulse depending on the instantaneous intensity of the video signal. The effect is comparable to that of a dot sequential system in which successive dot areas across the screens are excited to a degree depending on the picture values, the scanning line being shifted vertically to cover the entire screen area. Obviously, the greater the number of lines in the electrode grid, the finer the resolution of the picture.

While there has been shown what are considered to be preferred embodiments of the invention, it will be manifest that many changes and modifications may be made therein without departing from the essential spirit of the invention. It is intended, therefore, in the annexed claim to cover all such changes and modifications as fall within the true scope of the invention.

What is claimed is:

Apparatus for electrostatically recording information on a motionless insulating medium comprising first and second insulating plates, means to insert an insulating medium between said first and second sets of conductors for motionless electrostatic charge recording thereon and thereafter to remove the charged medium from between said sets for xerographic development, a first set of electrodes embedded in said first plate and constituted by a plurality of parallel wires extending in a given direction, a second set of electrodes embedded in said second plate and constituted by a plurality of parallel wires extending in a direction perpendicular to that of said first wires, a source of information constituted by a plurality of pulses, means to apply said pulses to respective wires of said first set, and means to apply a high voltage to a selected wire of said second set, the combined magnitude of said voltage and said pulse at a point of virtual intersection of said wires effecting a field discharge, said last named means including a ring counter having a plurality of stages connected in cascade relation, each stage being connected to a respective wire of said second set, means responsive to the successive applications of pulse information to the wires of the first set to successively activate the stages of said counter, said activated stage applying said high voltage to the associated wire.

References Cited by the Examiner UNITED STATES PATENTS 1,779,747 10/ 1930 Nicolson 178-6 2,426,079 8/ 1947 Bliss 340-324 2,558,019 6/1951 'Ioulon 178-75 2,610,102 9/1952 Gitzendanner 346-74 X 2,698,915 1/1955 Piper 315-362 2,716,048 8/ 1955 Young 346-74 2,869,965 1/ 1959 Willard 346-74 2,877,371 3/1959 Orthuber et al. 315-169 2,904,626 9/1959 Rajchman et a1 340-174 2,919,171 12/1959 Epstein 346-74 2,930,847 3/1960 Metzger 346-74 FOREIGN PATENTS 784,450 10/ 1957 Great Britain.

IRVING L. SRAGOW, Primary Examiner.

NEWTON N. LOVEWELL, STEPHEN W. CAPELLI,

ROBERT H. ROSE, Examiners. 

